Minimal incision removable bone screw

ABSTRACT

A surgical bone screw ( 1 ) and driver ( 31 ), and a method for using them for repairing an osteotomy or fracture. The screw has a shaft having a head ( 5 ) at its proximal end, screw threads ( 3 ) at its distal end and a compression member ( 7 ) between the head and the screw threads. In an embodiment, the threads are self-tapping and self-drilling, the head is polygonal, and the sides of the polygon are convex. The driver turns the screw until the threads cross a fracture site and the compression member contacts the proximal bone fragment. The threads and compression member draw the fragments together and leaves the head entirely clear of the bone surface. The screw is removed through a small incision. Tilting the driver to allow the socket to engage an inwardly extending lower portion of the head permits the driver to lift the screw out of the bone.

CROSS-REFERENCE TO RELATED APPLICATIONS

This continuation application claims the benefit of pending U.S.non-provisional application Ser. No. 14/233,866 filed Jan. 20, 2014,which is a United States national phase filing of PCT Application No.PCT/US12/047743 filed on Jul. 20, 2012 and is related to and claims thebenefit of U.S. provisional Applications No. 61/673,681, filed Jul. 19,2012, and No. 61/509,940, filed Jul. 20, 2011, the disclosures of whichare all hereby incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

TECHNICAL FIELD

The present invention relates to a bone screw, a driver for the screw,and a method of applying and removing the screw. The system may be usedas a removable fixation system for any osteotomy or fracture requiringlag compression. The screw has particular application forinterfragmentary internal fixation of small bones, such as those of thefoot, hand, and ankle, but is also useful with osteotomies or fracturesof large bones and spine.

BACKGROUND ART

When a bone is fractured, either by deliberately cutting it (osteotomy)or by trauma, it heals better if the bone fragments are pressed firmlytogether. Compression of the fragments increases the contact area acrossthe fracture and increases stability of the bone at the fracture. Italso decreases stress on any orthopedic implant.

Internal fixation of a bone fracture using bone screws is now commonpractice. The screw is applied across the fracture, preferably at nearlya right angle to the fracture, although the nature of the bone and ofthe fracture frequently dictates other angles. The distal end of thescrew crosses the fracture, and when the head of the bone screw engagesthe proximal fragment, further rotation of the screw draws the distalfragment of the bone against the proximal fragment. Any screw that isused to achieve interfragmental compression is termed a lag screw. Thetwo most common types of lag screws are cortical and cancellous screws.Cortical screws have fine threads on their shaft and are designed toanchor in cortical bone. Cancellous screws tend to have coarser threadsand are designed to anchor in the softer cancellous bone.

Both types of lag screw generally include a threaded distal end and aproximal head. Although the screw may be threaded nearly to the head,this design requires that the proximal bone fragment be pre-bored topermit the threads to pass smoothly through the proximal fragment. Morecommonly, the threads on the distal end extend only far enough to ensurea positive grip in the distal fragment but not so far as to engage theproximal fragment when the screw is applied, the shaft between thethreads and the head being smooth and sized no larger than the minordiameter of the threads (the maximum diameter of the thread groove). Thedistal side of the head, facing the shaft, is usually symmetricallyconvex, preferably hemispherical, and the proximal face of the proximalbone fragment is frequently lightly countersunk, in order to spreadstresses in the screw and the bone most efficiently, to reduce the riskof creating a stress fracture, and to minimize the protrusion of thescrew head from the face of the bone. The upper, proximal, side of thescrew head is generally flat or gently rounded to permit the head to lieas close to level with the proximal bone surface as possible. The headis provided with a slot, spaced holes, a hexagonal socket, or otherdepression to accept the blade or tip of a drive tool or screwdriverdesigned to be inserted into it.

In order to eliminate the need for pre-drilling bone and tapping thedistal bone fragment, bone screws are now frequently made to beself-tapping. To aid further in the placement of bone screws, the screwsare frequently cannulated, having a hollow shaft and head. Cannulatedscrews may be placed more precisely than non-cannulated screws. Thesurgeon first drills a small Kirschner wire (K-wire) across thefracture, generally under fluoroscopic control. The wire may sometimesbe inserted through the skin without the need of an incision. Ifnecessary, the K-wire can be withdrawn and replaced with minimal traumato the bone in order to place it in optimal position across thefracture. A small incision may then be made through the skin to enablethe surgeon to minimize tissue trauma while placing the bone screw andto permit countersinking the bone around the point of insertion of thescrew. The cannulated screw is then placed over the wire and slid downto the bone surface. A special cannulated driving tool then allows thescrew to be driven into the bone along the shaft of the K-wire. TheK-wire is then withdrawn and the wound over the screw is closed.

The construction and use of bone screws has become standardized to agreat extent. There are of course, many variations on the details of theconstruction of bone screws, including for example, the use of abreak-away driven element as shown in Patterson et al, U.S. Pat. No.8,221,478.

Bone screws may also be designed merely as an anchor for attaching anexternal stabilizing device. Pedicle screws, such as illustrated inMazda et al., U.S. published application US 2004/0116932 A1, areexamples of such external fixation screws. The present invention is notprincipally concerned with such screws, although some aspects of theinvention may be applicable to them.

Internal fixation bone screws may be left in the body afterimplantation. However, surgeons are increasingly removing fixation for anumber of justifiable reasons. Irritation/inflammation, allergicreaction, and infection are common reasons to remove hardware atappropriate times. Although rare, implant rejection may occur.Furthermore, the long-term deleterious effects of a metal such asstainless steel or a titanium alloy implanted in the body are not fullyunderstood. It is not uncommon for barometric pressure and changes inambient temperature to cause rheumatic or osteoarthritic flare-ups. Thepossibility of a causal relationship influenced by hardware left in andaround these areas exists. Finally, because of growing concerns overelectro-magnetic radiation caused by cell phone use and other exposure,it is ideal that conductive metals be removed from the body if possible.MRI and other present electromagnetic technologies are influenced byconductive, metallic implants. Future technologies may depend on thebody being free of conductive elements. Therefore, particularly when adeleterious effect is noted, a bone screw is sometimes removed, therebyallowing bone regeneration in the volume formerly occupied by the bonescrew. Such removal, however, requires considerable effort and risk, assuggested by patents such as Bonati et al., U.S. Pat. No. 7,090,680,Steffee, U.S. Pat. No. 4,854,311, Vasta et al., U.S. Pat. No. 7,582,093,or Lindemann et al., U.S. published application US 2007/0270880 A1.Special screw removal kits including multiple instruments arecommercially available.

When faced with having to remove a screw of the prior art, a surgeonmust deal with creeping fibrosis, meaning soft tissues that creep intothe screw threads making it difficult to access the screw without anincision into periosteal structures and more trauma. Fibrosis, in aworst-case scenario will make it necessary for a full screw extractionset to be utilized thereby completely bypassing the conventionalmethodologies of placing a screwdriver into a head. This usuallyrequires cutting the screw or severe countersinking.

In order to overcome these problems, some bone screws are made ofabsorbable materials. These screws, however, are not as strong as metalscrews, require drilling and tapping with metal instruments, and aretransparent to x-rays. Hybrid metal and polymer screws are disclosed inFischer et al., U.S. Pat. No. 4,711,232 and in TenHuisen et al., U.S.Pat. No. 6,916,321, but these screws add complexity and do not solve allof the problems with leaving metal in the body. Another solution hasbeen the use of screws made of compatible bone, as in Reed, U.S. Pat.No. 5,968,047. This approach is costly and has not been entirelysatisfactory.

BRIEF SUMMARY OF THE INVENTION

Briefly stated, the present invention provides a bone screw which iseasily placed and easily removed, usually after bone healing. The bonescrew may be placed and removed using a simple screwdriver in accordancewith the invention.

In accordance with one embodiment, a bone screw comprises a thread atits distal end, a head at its proximal end, and a bone-engagingcompression member spaced distally from the head. The screw may be madeof any biocompatible material, but is preferably made of stainlesssteel, titanium, or titanium alloy.

Preferably, the thread does not extend as far as the compression member.Preferably, the thread is self-tapping. In some situations it may beself-drilling, although this is not presently preferred. A thread whichis neither self-tapping nor self-drilling is also useable. Many types ofthreads are known and are useable. Both cortical and cancellous screwthreads may be used. If the threads extend to or nearly to thecompression member, the proximal bone fragment must be pre-drilled to adiameter as great as the thread major diameter to allow the compressionmember to create a lag effect on the fracture. The depth and shape ofthe threads, and their length may be established in accordance withknown parameters.

Preferably, the bone-engaging compression member is convex on its distalside and is circular in cross-section. Preferably the compression memberis convex on its proximal side and has a smoothly curved outline withoutedges to permit easy extraction through an incision and to provide aneck between the head and the compression member. A ball having adiameter from 10% to 70% greater than the screw diameter is preferred. Aball diameter of 110% to 140% of the maximum major thread diameter or adiameter of 125% to 160% of the shaft diameter is particularly suitable.

The head is preferably non-circular as viewed in top plan elevation(end-on), so that its periphery may be drivingly engaged by a driver.Also preferably, at least two opposed sides of the head are curvedinward toward both the proximal end of the screw and the distal end ofthe screw to enable a hollow head of a screw driver to engage the screwhead to transmit sufficient torque to drive screw into or out of bonewhen the driver is tilted at angles of up to fifteen degrees or possiblytwenty degrees from the axis of the screw (+/−15°-20° articulation). Theinwardly curved distal faces of the head also permit the screw to bedrawn out of the bone and incision when the driver is articulated awayfrom the axis of the screw. In a preferred embodiment, the head is inend view a regular polygon having an even number of sides, preferablyfour or six. With the polygonal configuration, it is convenient for thedistance between opposite sides to be about equal to the major threaddiameter, ±5%. Thus, for example, a two millimeter screw (having a majorthread diameter of two millimeters) may have a two millimeter squarehead, and a five millimeter screw may have a five millimeter squarehead. The height of the head, in accordance with the geometry of itssides, is generally about the same as its width, typically on the orderof 80% to 100% of its width, usually 90% to 95% of the major threaddiameter.

The spacing distance between the base of the head and the widest part ofthe compression member may be chosen to suit the use of the screw. Forsome applications, the head is placed just below the skin, withoutcausing the skin to “tent”, so it can be found easily by subcutaneouspalpation and removed easily with minimal trauma to surrounding tissue.For these applications, a screw having a spacing distance of about threeto nine millimeters may be used. Because the screw will generally beremoved in a few weeks, accessibility of the screw for ease of removaldictates that in many situations a screw having a long spacing be placedso that the head extends into a concavity below the surface of theepidermis. For other applications, patient comfort or anatomy demands ahead that is closer to the bone surface. For these applications aspacing distance of one to three millimeters may be used, and if thescrew cannot be found by palpation, it may be found by mechanical ormachine means, such as by fluoroscopy.

In most instances, the axial distance from the top (proximal end) of thehead to the widest part of the compression member is greater than thelargest diameter of the head.

In preferred embodiments, the head and compression member are formedwith no sharp edges. All edges are rounded as much as is consistent withmaintaining a non-circular head which may be driven by a socketextending over the head and engaging its periphery, so as to maintainthe lowest practical coefficient of friction (interaction) between thescrew and surrounding soft tissue including nerves and blood vesselsafter the driver is removed. The top (proximal end) of the head may beflat to minimize height above the working sides of the head, or it maybe convex, even spherical, to minimize friction with surrounding tissueand nerves (neuropraxia).

The screw may be either solid or cannulated. When cannulated, the sizeof the cannula may be chosen in accordance with the size of the wire orpin. K-wire sizes from about 0.7 mm to about 1.6 mm are common.Illustrative screws of the present invention have cannulae whichtypically range from about 0.75 mm for a two millimeter diameter screwto about 1.4 mm for a seven millimeter diameter screw.

A set of screws of the present invention may include several families ofscrews of different diameters, for example 2.0, 2.5, 3.0, 3.5, 4.0, 5.0,6.0, and 7.0 mm. Each family may include screws of different lengths,measured from the distal end of the screw to the maximum diameter of thecompression member, ranging, say, from 8 mm to 60 mm in two millimeterincrements. Each length of each family may in turn include differentcompression member-to-head base dimensions, say 1.0 mm, 3.5 mm, 5.0 mm,and 8.0 mm. Because the size of the screw head in each family isconstant, the overall length of the screws of a given nominal length mayvary based on the length of the compression member to head spacing. Itis anticipated that screws of the invention will be packaged and sold insets including at least different compression member-to-head basedimensions.

The screw driver of the invention may comprise a simple open box socketat the distal end of a driver shaft axially aligned with a handle. Theinside walls of the socket may be parallel and form a shapecomplementary to the outside dimensions of the screw head, withdimensions just sufficiently larger than those of the exterior of thehead to enable the socket to slip easily over the head while maintaininga positive contact with the exterior faces of the head. A spacing onehalf percent to ten percent larger than the head width at its greatestwidth between parallel faces is suitable. The depth of the socket ispreferably equal to the depth of the head ±10%. The exterior of thesocket is smooth, and the wall thickness of the socket is as thin as isconsistent with strength, to minimize the amount an incision must bespread to accommodate the driver. The exterior of the socket may beround to minimize interference with surrounding tissue as the driver isrotated, or the corners of a polygonal exterior may be rounded. Thehandle and socket may be formed as a single piece, or they may beseparate pieces which are permanently or removably connected to eachother. The socket and its shaft are preferably cannulated to accommodatecannulated screws. The socket and the shaft of the driver are preferablymade of stainless steel, although titanium, titanium alloys, and othermaterials are useable.

Where the handle and socket are not removably connected, a kit ofscrewdrivers would consist of a screwdriver for each diameter of screw,illustratively a 2.0, 2.5, 3.0, 3.5, 4.0, 5.0, 6.0, and 7.0 mm size.Where they are removably connected, a single handle and appropriatesockets make up the kit. It will be understood that the shaft of thescrewdriver could be made a part of the handle or a part of theremovable socket, preferably of the socket.

In accordance with the method of the invention, a screw of the inventionis inserted transfragmentally through a fracture, preferably at an angleto the fracture chosen for acceptably compressing the bone fragments inaccordance with standard practice, and in order to place the head in anaccessible position that will provide minimum patient discomfort. Thescrew is tightened into the distal bone fragment using the screwdriverof the invention, to place the compression member in contact with theproximal bone fragment and to draw the bone fragments into snug contact.Unlike many previously-known screws which are not specifically designedfor removal, the screw of the present invention may, if desired, bedriven through and beyond the margin of the distal bone fragment. Thisexit site is generally spaced far enough from the epidermis to preventthe production of a secondary wound. Screwing the internal fixation bonescrew through the distal bone ensures that the screw is secured incortical bone and is therefore less likely to lose purchase.

The tissue over the insertion site is then closed. When soft tissueclosure over the screw involves capsule and/or subcutaneous layers, thecapsule and layers can be closed over the screw head, a small incisionmade, and the layers pushed down below the head and over the compressionmember before closing the epidermis. On removal of the screw, only theepidermis needs to be incised to expose the screw head for removal ofthe screw. In most cases, further incision of the subcutaneous layers isnot needed. This ease of removal is a major advantage of the presentinvention.

Previously, those concerned with internal fixation bone screws havebelieved that minimizing the protrusion of the head from the surface ofthe bone to which it is affixed is of great importance. See for exampleZang, U.S. Pat. No. 5,556,225. In accordance with the present invention,accessibility of the head and ease of removing an internal fixationscrew have been found to be more important. An additional advantage tohaving removable hardware that is easy to remove is that it is equallyeasy to replace. This means that should a screw lose purchase, a largerscrew can serve as a replacement screw without the necessity of removalthrough a large incision and reevaluation of the exact size.

After the bone fracture has healed sufficiently, in accordance withradiographic and clinical evidence (frequently two to eight weeks afterthe procedure), the head of the screw is located either by palpation orby other means such as needle or fluoroscopy, and a small incision ismade to expose the screw head.

Particularly if the screw head is near the surface, a small stabincision may be sufficient to access the screw head. The head of thescrew may be cleaned, although this step is far less important than witha screw head relying on a depression in the top of the head forengagement with the driver. The screwdriver is placed over the head.Because the head is separate from the compression member, it is easilyreached, and because the head is undercut, tilting the screwdriverallows it to exert a gentle outward force on the screw. The screwdriveris used for turning and removing the screw from the bone. The roundedshape of the head allows positive contact between the side walls of thedriver socket and the head throughout a range of angles of at leastfifteen degrees. Therefore, the driver may be tilted a few degrees fromthe axis of the screw, preferably around ten to twenty degrees, and usedfor gently lifting the screw.

When the screw is clear of the bone, it may be removed with forceps. Thesmall incision is closed, preferably after bathing it. The bone is thenallowed to regrow into the cavity left by the screw. It will be seenthat the screw head will frequently be so close to the epidermis andwill be so completely exposed by even a small incision, that removal ofthe screw may not even be dependent on the special qualities of thescrewdriver.

The screw, driver, and methods of the present invention are particularlywell suited to surgery on the human foot and hand. Examples arefractures of the phalanges, metatarsals, or talus of the foot, fracturesof the phalanges or scaphoid of the hand, and in osteotomies of the handand foot such as bunion repair through a modified Scarf/Akin osteotomyor a repair of a Jones fracture of the hand. It will be understood thatosteotomies may involve both the removal of bone and the insertion ofbone between bone fragments, so that the screw may pass through morethan one proximal bone fragment. It will also be understood that thescrew, driver, and methods of the invention are applicable to a widerange of other procedures on human and non-human vertebrate bones,including for example, other osteotomies and fractures of the hip, leg,arm, spine, or clavicle. The screw, driver and methods may also beuseable with non-metallic, biocompatible, and bioabsorbable platingdevices to avoid long-term exposure to metal components.

The foregoing and other objects, features, and advantages of theinvention as well as presently preferred embodiments thereof will becomemore apparent from the reading of the following description inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the accompanying drawings which form part of the specification:

FIG. 1. is a view in perspective of one illustrative screw according toan embodiment of the present invention.

FIG. 2 is a view in side elevation of the screw of FIG. 1.

FIG. 3 is a top plan view of the screw of FIGS. 1 and 2.

FIG. 4 is a view in axial cross-section of the screw of FIGS. 2-3.

FIG. 5 is a detail in axial cross-section of a head part of the screw ofFIGS. 2-4.

FIG. 6 is a detail view of a distal tip part of the screw of FIGS. 1-5.

FIG. 7 is a detail view in axial cross-section of a screw thread part ofthe screw of FIGS. 1-6.

FIG. 8 is a diagonal cross section taken along the line 8-8 of FIG. 3.

FIG. 9 shows a family of screws of the present invention, each screwhaving the same nominal length, but with different overall lengthsdetermined by a head spacing dimension “B.”

FIG. 10 shows a family of screws of the present invention, one group ofscrews having a “long” head spacing dimension “B” and a second grouphaving a short head spacing dimension “B”, each screw in each grouphaving different nominal lengths and different thread lengths “C”.

FIG. 11 is a view in side elevation of a screw driver according to anembodiment of the present invention.

FIG. 12 is an end view of the driver of FIG. 11.

FIG. 13 is a fragmentary axial cross-section of a socket portion of thedriver of FIGS. 11 and 12, taken along the line 13-13 of FIG. 12.

FIG. 14 is an end view of the fragment of FIG. 13.

FIG. 15 is a view in side elevation of a handle part of the driver.

FIG. 16 is an end view of the handle of FIG. 15.

FIG. 17 is a fragmentary sectional view taken along line 17-17 of FIG.16.

FIGS. 18 and 19 are schematic views of the fixation of a fracture withthe screw 1 and driver 31 of the invention.

FIG. 20 is a cross-sectional view corresponding to FIG. 4 of anotherembodiment of bone screw.

FIG. 21 is a cross-sectional view corresponding to FIGS. 4 and 19 of yetanother embodiment of bone screw.

Corresponding reference numerals indicate corresponding parts throughoutthe several figures of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following detailed description illustrates the invention by way ofexample and not by way of limitation. The description clearly enablesone skilled in the art to make and use the invention, describes severalembodiments, adaptations, variations, alternatives, and uses of theinvention, including what is presently believed to be the best mode ofcarrying out the invention.

As shown in FIGS. 1-8, in accordance with one embodiment, a bone screw 1comprises a thread 3 at its distal end, a head 5 at its proximal end,and a bone-engaging compression member 7 spaced distally from the head.The screw 1 may be made of any biocompatible material, but is preferablymade of stainless steel, titanium, or titanium alloy. In thisillustrative embodiment, it is made of the titanium alloy known asTi6Al4V ELI, with an anodized finish in accordance with SAE AMS2488D.

The screw 1 is first identified by its diameter and by its length, as ina standard lag bone screw. Diameter is defined as the major diameter ofthe screw thread 3, in this illustrative embodiment 2.0 mm. The length Aof the screw is measured from its distal end 9 to the largest diameterof the compression member 5. The size of the head 5 is nominally thesame both across from face to face as it is tall (top to bottom), andboth these dimensions are nominally the same as the screw diameter(major thread diameter). In this embodiment, the head is 2.0 mm acrossand 1.9 mm tall. Unique to the screw of the invention is a dimension Bmeasured from the largest diameter of the compression member 5 to thebase 11 of the head 5, as discussed more fully hereinafter. The minordiameter of the thread 3 is equal to the shaft diameter of the screwshaft 13 between the thread 3 and the compression member 7 and betweenthe compression member 7 and the head base 11. That dimension in thisembodiment is 0.75 times the major thread diameter, or 1.5 mm. Thethread 9 has a length C that varies with the length of the screw 1. Fora 16.0 mm long screw, the thread has a length of about 6 mm.

The thread 3 is self-tapping, but requires a pilot hole of about thediameter of the shaft 13.

The bone-engaging compression member 7 has a radius from about 1.1 toabout 1.25 times the diameter of the screw. In this illustrativeembodiment, the compression member 7 is a sphere having a diameter of2.2 mm.

The head 5 in this illustrative embodiment is generally in the form of acube having a side nominally equal to the screw diameter. The top planview (FIG. 3) of the head 5 shows the sides as straight, but as viewedin side elevation (FIG. 2) or in cross-section (FIGS. 4 and 5), thesides are sloped inward top and bottom at an angle of 16° from a maximumconvex dimension 15, as indicated at 17 and 19 respectively.

The spacing distance B between the base 11 of the head 15 and the widestpart of the compression member may be chosen to suit the use of thescrew. In this illustrative embodiment, as shown in FIG. 9, a set 23 of“long head” screws have spacing distance B of 3.5 mm, 5.0 mm, and 8.0mm, respectively. For applications in which little room is availablebelow the skin, a “short head” having a spacing distance of 1.0 mm, isprovided.

The head 5 and compression member 7 are formed to be smooth, with nosharp edges.

The illustrative screw is cannulated, having a central bore 21 of 0.75(+0.05) mm. A solid screw would look the same, but without the centralcannula.

As indicated in FIG. 10, each family includes screws of differentlengths, measured from the distal end of the screw to the maximumdiameter of the compression member, ranging from 12 mm to 60 mm in twomillimeter increments. Each length of each family in turn includesdifferent compression member-to-head base dimensions “B”: 1.0 mm (“shorthead”), 3.5 mm, five millimeters, and eight millimeters. Because thesize of the screw head in each family is constant, the overall length ofthe screws of a given nominal length vary based on the length of thecompression member to head spacing. As shown in FIG. 10, differentlengths of screw 1 will have different thread lengths “C”.

An illustrative screw driver 31 of the invention is shown in FIGS.11-17. The driver 31 comprises a simple open box socket 33 formedintegrally at the distal end of a driver shaft 35 axially aligned with ahandle 37, as shown in FIGS. 11-17. The shaft 35 and socket 33 have anoverall length of about 125 mm. The shaft and socket are formed from asingle 0.375 mm diameter rod of 17-4PH H900 stainless steel andpassivated per ASTM A967. At its distal end, the round tube is squaredand routered to form inside walls 39 of the socket. The inside surfacesof the walls 39 are flat and parallel and have a side “D” of2.07+/−0.01, just larger than the sides of the screw head 5. The depthof the socket is 2.0 mm, about 0.1 mm deeper than the height of thescrew head. A shallow well 41 at the bottom of the socket 33 acts as aguide for a K-wire to enter a 0.90+/−0.05 mm cannula 43 extendingthrough the driver shaft 35. The exterior of the socket 33 is smooth,and the wall thickness of the socket is as thin as is consistent withstrength, to minimize the amount an incision must be spread.

The shaft 35 of the driver 31 has welded to its proximal end a standardadapter 45 sized to fit a hollow 47 in the handle 37, to which it isattached. The handle 37 is formed of polyphenylsulfone (Radel® R5500,Solvay Advanced Polymers L.L.C). In this embodiment, the handle 37,shaft 35, and socket 33 are packaged as a single unit, with handles ofdifferent colors signifying different socket sizes.

A kit of screwdrivers in this embodiment consists of eight screwdrivers,each with a handle 37 secured to a shaft/socket of an appropriate sizefor each diameter of screw, in this illustrative embodiment a 2.0, 2.5,3.0, 3.5, 4.0, 5.0, 6.0, and 7.0 mm size.

An example of the use of the screw, driver, and method of the presentinvention to treat a bunion using the Akin procedure was conducted asfollows:

A chevron style osteotomy was made through and into the head of thefirst metatarsal.

Next, a 0.062 Kirschner wire was utilized to drill a pilot hole throughboth sides of the osteotomy.

Next, a depth gauge was utilized to determine the appropriate screwlength.

Next a countersink was performed at the proximal entry point of thek-wire.

Next, an appropriately sized bone screw was screwed into the osteotomyand compression was noted and achieved to two finger tightness.

Next capsular closure was performed over the head of the screw creatinga mild tenting effect directly over this.

Upon the completion of capsular closure the screw head was palpated anda small stab incision was made into the capsule whereby the head of thescrew was pushed through the capsule exposing the head.

Next the sub-cuticular layer was closed once again over the head of thescrew creating a mild tenting.

Upon the completion of the sub-cuticular closure the screw head wasagain palpated and a small stab incision was made to expose the screwhead through this layer.

Finally, the epidermal layer was closed utilizing absorbable 5-0 Vicrylsuture. On completion of this closure it is noted that there is notenting of the epidermis due to screw head prominence. Palpation of thescrew head can, however, be appreciated through the epidermis.

After an appropriate period, removal was conducted as follows:

The patient was brought into the operating room and prepped and drapedin the usual aseptic manner.

Next, attention was directed to the dorsal aspect of the foot which waspalpated locating the head of the previously applied cortical removablebone screw.

Next, approximately 1.5 mL of lidocaine one percent was utilized toachieve anesthesia.

Next, a stab incision was made with a number eleven blade directly overthe head of the previously mentioned screw.

Next, the driver was placed into the wound and the screw head waslocated and securely contact fitted around the driver head. The driverwas tilted at approximately 15° to create the appropriate pulling effectas the screw was removed. Once the screw head was noted to exit thesmall epidermal incision, a small hemostat was utilized to secure theskin around the screw head and allow further secure removal of thescrew.

Upon complete removal, the remaining incision was closed with a single5.0 nylon suture.

Another example of the use of the present invention is shownschematically in FIGS. 18 and 19. An incision is made through theepidermis 57 and subdermal tissue 59, and they are retracted. Thefracture in bone 53 is reduced by manipulation, and drilled with aK-wire through both a proximal fragment 56 and a distal fragment 54 atan angle to the fracture chosen for acceptably compressing the bonefragments in accordance with standard practice, and in order to placethe head in an accessible position that will provide minimum patientdiscomfort. The K-wire is removed, and the bone fragment 56 is mildlycountersunk. The depth of the hole is measured with a depth gauge. Aproper size cannulated screw 1 is chosen based on angle of entry andconcavity of bone surface and distance from bone surface to skinsurface. A K-wire having both ends tapered is inserted, and the screw 1is tightened over it to two finger tightness and good compression usingthe screwdriver 31. The K-wire is removed. The capsule 59 is closed overthe head 5, tenting the capsule. The capsule is then incised directlyover the screw head 5 to expose the head. Any subcutaneous tissue islikewise closed over the head 5, incised to expose the head, and closed.The epidermis 57 is then closed and sutured as indicated at 61.

The screw 1 is removed in the same way as in the previous example,requiring only a small stab incision to expose the screw head 5 andallow removal of the screw.

Numerous variations, within the scope of the appended claims will occurto those skilled in the art in light of the foregoing description.Merely by way of example, although standard thread count and spacingwill typically be used, the thread count, spacing, or both may bechanged from screw to screw, without departing from the scope of thepresent invention. The shape of the head and its spacing from thecompression member may be varied widely.

The head may even be made with a conventional hex socket and driven witha conventional hex-head driver as shown in FIG. 20, corresponding toFIG. 4. This approach allows a slightly more rounded head, but itsuffers from the problems of ingrowth into the hex socket and possibledifficulties in removing the screw. Leaving a neck below the head andabove the compression member, however, is highly advantageous inproviding purchase for aiding in the extraction of the screw. The slightangulation of the driver allows for the retrograde force frequentlyneeded to remove the screw.

As shown in FIG. 21, it is even possible to obtain some of theadvantages of the present invention by melding the compression ball andthe head of the embodiment of FIGS. 1-8 into a single body. Thatapproach does elevate the head to a more reachable position, but it isbelieved to lack most of the other advantages of the illustrativeembodiments.

Screw head shape can be square, triangular, rectangular, oblong, or anyother shape that will facilitate this process. It is believed at presentthat having at least two convex opposed faces is advantageous forpositive driving of the screw in both directions and for lifting thescrew as it is removed. The shape of the compression member may also bevaried. A ball shape, whether spherical or flattened, is preferredbecause of its lack of edges, and because it distributes stressesefficiently. The size of the compression member may be varied; it isbelieved that a somewhat larger ball, perhaps one millimeter larger thanpresently preferred, may give somewhat improved results.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results are obtained. Asvarious changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

All of the patents, patent applications, and literature referencesmentioned herein are hereby incorporated by reference.

1. A bone screw for repairing an osteotomy or fracture by lagcompression, the bone screw comprising: a shaft having a proximal endand a distal end; a head at the proximal end of the shaft, the headhaving a proximal end; a screw thread at the distal end of the shaft; anenlarged bone-engaging convex compression surface disposed on the shaftbetween the proximal end of the head and the screw thread, characterizedin that the head and the compression surface are formed to be smoothwith no sharp edges, a distance between the proximal end of the head anda distal end of the compression surface being at least twice a majordiameter of the screw thread, and in that the head and the compressionmember are configured for placement of the head below the skin of apatient when the screw is inserted across the osteotomy or fracture. 2.The bone screw of claim 1 wherein the enlarged surface is a distalportion of the head.
 3. The bone screw of claim 1 wherein the head hasan axially extending socket at its proximal end.
 4. The bone screw ofclaim 3 wherein the socket is polygonal.
 5. The bone screw of claim 1wherein a proximal end of the head is polygonal.
 6. The bone screw ofclaim 1 wherein the screw is cannulated.
 7. A bone screw made of abiocompatible material, the bone screw being sized and shaped to fixbone fragments of an osteotomy or fracture of a bone by screwing into adistal fragment and engaging a proximal fragment with a compressionsurface on the screw, the head and the compression member beingconfigured for placement of the head below the skin of a patient, thescrew further being configured to be removed after bone healing hasprogressed, the bone screw comprising: a thread at a distal end of thescrew, the thread having a major diameter and a minor diameter; a headat a proximal end of the screw; a bone-engaging compression surfacespaced distally from a proximal end of the head, the bone-engagingcompression member being convex on a distal side and circular incross-section, and having a smooth outline without edges to permit easyinsertion and extraction through an incision, the compression surfacehaving a maximum diameter from 10% to 70% greater than the majordiameter of the thread, a distance from the proximal end of the head tothe distal end of the compression surface being from two to six timesthe major diameter of the thread.
 8. The bone screw of claim 7 whereinthe enlarged surface is a distal portion of the head.
 9. The bone screwof claim 7 wherein the head has an axially extending socket at itsproximal end.
 10. The bone screw of claim 9 wherein the socket ispolygonal.
 11. The bone screw of claim 7 wherein a proximal end of thehead is polygonal.
 12. The bone screw of claim 7 wherein the screw iscannulated.